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Network Working Group                                          M. GahrnsRequest for Comments: 2180                                     MicrosoftCategory: Informational                                        July 1997IMAP4 Multi-Accessed Mailbox PracticeStatus of this Memo   This memo provides information for the Internet community.  This memo   does not specify an Internet standard of any kind.  Distribution of   this memo is unlimited.1. Abstract   IMAP4[RFC-2060] is rich client/server protocol that allows a client   to access and manipulate electronic mail messages on a server.   Within the protocol framework, it is possible to have differing   results for particular client/server interactions. If a protocol does   not allow for this, it is often unduly restrictive.   For example, when multiple clients are accessing a mailbox and one   attempts to delete the mailbox, an IMAP4 server may choose to   implement a solution based upon server architectural constraints or   individual preference.   With this flexibility comes greater client responsibility.  It is not   sufficient for a client to be written based upon the behavior of a   particular IMAP server.  Rather the client must be based upon the   behavior allowed by the protocol.   By documenting common IMAP4 server practice for the case of   simultaneous client access to a mailbox, we hope to ensure the widest   amount of inter-operation between IMAP4 clients and servers.   The behavior described in this document reflects the practice of some   existing servers or behavior that the consensus of the IMAP mailing   list has deemed to be reasonable.  The behavior described within this   document is believed to be [RFC-2060] compliant. However, this   document is not meant to define IMAP4 compliance, nor is it an   exhaustive list of valid IMAP4 behavior. [RFC-2060] must always be   consulted to determine IMAP4 compliance, especially for server   behavior not described within this document.Gahrns                       Informational                      [Page 1]

RFC 2180         IMAP4 Multi-Accessed Mailbox Practice         July 19972. Conventions used in this document   In examples,"C1:", "C2:" and "C3:" indicate lines sent by 3 different   clients (client #1, client #2 and client #3) that are connected to a   server.  "S1:", "S2:" and "S3:" indicated lines sent by the server to   client #1, client #2 and client #3 respectively.   A shared mailbox, is a mailbox that can be used by multiple users.   A multi-accessed mailbox, is a mailbox that has multiple clients   simultaneously accessing it.   A client is said to have accessed a mailbox after a successful SELECT   or EXAMINE command.   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",   "SHOULD", "SHOULD NOT", "RECOMMENDED",  "MAY", and "OPTIONAL" in this   document are to be interpreted as described in [RFC-2119].3. Deletion/Renaming of a multi-accessed mailbox   If an external agent or multiple clients are accessing a mailbox,   care must be taken when handling the deletion or renaming of the   mailbox. Following are some strategies an IMAP server may choose to   use when dealing with this situation.3.1. The server MAY fail the DELETE/RENAME command of a multi-accessed     mailbox   In some cases, this behavior may not be practical.  For example, if a   large number of clients are accessing a shared mailbox, the window in   which no clients have the mailbox accessed may be small or non-   existent, effectively rendering the mailbox undeletable or   unrenamable.   Example:   <Client #1 and Client #2 have mailbox FOO accessed. Client #1 tries   to DELETE the mailbox and is refused>             C1: A001 DELETE FOO             S1: A001 NO Mailbox FOO is in use by another user.Gahrns                       Informational                      [Page 2]

RFC 2180         IMAP4 Multi-Accessed Mailbox Practice         July 19973.2. The server MAY allow the DELETE command of a multi-accessed     mailbox, but keep the information in the mailbox available for     those clients that currently have access to the mailbox.   When all clients have finished accessing the mailbox, it is   permanently removed.  For clients that do not already have access to   the mailbox, the 'ghosted' mailbox would not be available.  For   example, it would not be returned to these clients in a subsequent   LIST or LSUB command and would not be a valid mailbox argument to any   other IMAP command until the reference count of clients accessing the   mailbox reached 0.   In some cases, this behavior may not be desirable. For example if   someone created a mailbox with offensive or sensitive information,   one might prefer to have the mailbox deleted and all access to the   information contained within removed immediately, rather than   continuing to allow access until the client closes the mailbox.   Furthermore, this behavior, may prevent 'recycling' of the same   mailbox name until all clients have finished accessing the original   mailbox.   Example:   <Client #1 and Client #2 have mailbox FOO selected. Client #1 DELETEs   mailbox FOO>             C1: A001 DELETE FOO             S1: A001 OK Mailbox FOO is deleted.   <Client #2 is still able to operate on the deleted mailbox>             C2: B001 STORE 1 +FLAGS (\Seen)             S2: * 1 FETCH FLAGS (\Seen)             S2: B001 OK STORE completed   <Client #3 which did not have access to the mailbox prior to the   deletion by client #1 does not have access to the mailbox>             C3: C001 STATUS FOO (MESSAGES)             S3: C001 NO Mailbox does not exist   <Nor is client #3 able to create a mailbox with the name FOO, while   the reference count is non zero>             C3: C002 CREATE FOO             S3: C002 NO Mailbox FOO is still in use. Try again later.Gahrns                       Informational                      [Page 3]

RFC 2180         IMAP4 Multi-Accessed Mailbox Practice         July 1997   <Client #2 closes its access to the mailbox, no other clients have   access to the mailbox FOO and reference count becomes 0>             C2: B002 CLOSE             S2: B002 OK CLOSE Completed   <Now that the reference count on FOO has reached 0, the mailbox name   can be recycled>             C3: C003 CREATE FOO             S3: C003 OK CREATE Completed3.3. The server MAY allow the DELETE/RENAME of a multi-accessed     mailbox, but disconnect all other clients who have the mailbox     accessed by sending a untagged BYE response.   A server may often choose to disconnect clients in the DELETE case,   but may choose to implement a "friendlier" method for the RENAME   case.   Example:   <Client #1 and Client #2 have mailbox FOO accessed. Client #1 DELETEs   the mailbox FOO>             C1: A002 DELETE FOO             S1: A002 OK DELETE completed.   <Server disconnects all other users of the mailbox>             S2: * BYE Mailbox FOO has been deleted.3.4. The server MAY allow the RENAME of a multi-accessed mailbox by     simply changing the name attribute on the mailbox.   Other clients that have access to the mailbox can continue issuing   commands such as FETCH that do not reference the mailbox name.   Clients would discover the renaming the next time they referred to   the old mailbox name.  Some servers MAY choose to include the   [NEWNAME] response code in their tagged NO response to a command that   contained the old mailbox name, as a hint to the client that the   operation can succeed if the command is issued with the new mailbox   name.Gahrns                       Informational                      [Page 4]

RFC 2180         IMAP4 Multi-Accessed Mailbox Practice         July 1997   Example:   <Client #1 and Client #2 have mailbox FOO accessed. Client #1 RENAMEs   the mailbox.>             C1: A001 RENAME FOO BAR             S1: A001 OK RENAME completed.   <Client #2 is still able to do operations that do not reference the   mailbox name>             C2: B001 FETCH 2:4 (FLAGS)             S2: * 2 FETCH . . .             S2: * 3 FETCH . . .             S2: * 4 FETCH . . .             S2: B001 OK FETCH completed   <Client #2 is not able to do operations that reference the mailbox   name>             C2: B002 APPEND FOO {300} C2: Date: Mon, 7 Feb 1994             21:52:25 0800 (PST) C2: . . .  S2: B002 NO [NEWNAME FOO             BAR] Mailbox has been renamed4. Expunging of messages on a multi-accessed mailbox   If an external agent or multiple clients are accessing a mailbox,   care must be taken when handling the EXPUNGE of messages.  Other   clients accessing the mailbox may be in the midst of issuing a   command that depends upon message sequence numbers.  Because an   EXPUNGE response can not be sent while responding to a FETCH, STORE   or SEARCH command, it is not possible to immediately notify the   client of the EXPUNGE.  This can result in ambiguity if the client   issues a FETCH, STORE or SEARCH operation on a message that has been   EXPUNGED.4.1. Fetching of expunged messages   Following are some strategies an IMAP server may choose to use when   dealing with a FETCH command on expunged messages.Gahrns                       Informational                      [Page 5]

RFC 2180         IMAP4 Multi-Accessed Mailbox Practice         July 1997   Consider the following scenario:   - Client #1 and Client #2 have mailbox FOO selected.   - There are 7 messages in the mailbox.   - Messages 4:7 are marked for deletion.   - Client #1 issues an EXPUNGE, to expunge messages 4:74.1.1. The server MAY allow the EXPUNGE of a multi-accessed mailbox but       keep the messages available to satisfy subsequent FETCH commands       until it is able to send an EXPUNGE response to each client.   In some cases, the behavior of keeping "ghosted" messages may not be   desirable.  For example if a message contained offensive or sensitive   information, one might prefer to instantaneously remove all access to   the information, regardless of whether another client is in the midst   of accessing it.   Example:  (Building upon the scenario outlined in 4.1.)   <Client #2 is still able to access the expunged messages because the   server has kept a 'ghosted' copy of the messages until it is able to   notify client #2 of the EXPUNGE>             C2: B001 FETCH 4:7RFC822             S2: * 4 FETCHRFC822 . . . (RFC822 info returned)             S2: * 5 FETCHRFC822 . . . (RFC822 info returned)             S2: * 6 FETCHRFC822 . . . (RFC822 info returned)             S2: * 7 FETCHRFC822 . . . (RFC822 info returned)             S2: B001 OK FETCH Completed   <Client #2 issues a command where it can get notified of the EXPUNGE>             C2: B002 NOOP             S2: * 4 EXPUNGE             S2: * 4 EXPUNGE             S2: * 4 EXPUNGE             S2: * 4 EXPUNGE             S2: * 3 EXISTS             S2: B002 OK NOOP Complete   <Client #2 no longer has access to the expunged messages>             C2: B003 FETCH 4:7RFC822             S2: B003 NO Messages 4:7 are no longer available.Gahrns                       Informational                      [Page 6]

RFC 2180         IMAP4 Multi-Accessed Mailbox Practice         July 19974.1.2 The server MAY allow the EXPUNGE of a multi-accessed mailbox,      and on subsequent FETCH commands return FETCH responses only for      non-expunged messages and a tagged NO.   After receiving a tagged NO FETCH response, the client SHOULD issue a   NOOP command so that it will be informed of any pending EXPUNGE   responses.  The client may then either reissue the failed FETCH   command, or by examining the EXPUNGE response from the NOOP and the   FETCH response from the FETCH, determine that the FETCH failed   because of pending expunges.   Example:  (Building upon the scenario outlined in 4.1.)   <Client #2 attempts to FETCH a mix of expunged and non-expunged   messages.  A FETCH response is returned only for then non-expunged   messages along with a tagged NO>             C2: B001 FETCH 3:5 ENVELOPE             S2: * 3 FETCH ENVELOPE . . . (ENVELOPE info returned)             S2: B001 NO Some of the requested messages no longer exist   <Upon receiving a tagged NO FETCH response, Client #2 issues a NOOP   to be informed of any pending EXPUNGE responses>             C2: B002 NOOP             S2: * 4 EXPUNGE             S2: * 4 EXPUNGE             S2: * 4 EXPUNGE             S2: * 4 EXPUNGE             S2: * 3 EXISTS             S2: B002 OK NOOP Completed.   <By receiving a FETCH response for message 3, and an EXPUNGE response   that indicates messages 4:7 have been expunged, the client does not   need to re-issue the FETCH>Gahrns                       Informational                      [Page 7]

RFC 2180         IMAP4 Multi-Accessed Mailbox Practice         July 19974.1.3 The server MAY allow the EXPUNGE of a multi-accessed mailbox, and      on subsequent FETCH commands return the usual FETCH responses for      non-expunged messages, "NIL FETCH Responses" for expunged      messages, and a tagged OK response.   If all of the messages in the subsequent FETCH command have been   expunged, the server SHOULD return only a tagged NO.  In this case,   the client SHOULD issue a NOOP command so that it will be informed of   any pending EXPUNGE responses.  The client may then either reissue   the failed FETCH command, or by examining the EXPUNGE response from   the NOOP, determine that the FETCH failed because of pending   expunges.   "NIL FETCH responses" are a representation of empty data as   appropriate for the FETCH argument specified.   Example:   * 1 FETCH (ENVELOPE (NIL NIL NIL NIL NIL NIL NIL NIL NIL NIL))   * 1 FETCH (FLAGS ())   * 1 FETCH (INTERNALDATE "00-Jan-0000 00:00:00 +0000")   * 1 FETCH (RFC822 "")   * 1 FETCH (RFC822.HEADER "")   * 1 FETCH (RFC822.TEXT "")   * 1 FETCH (RFC822.SIZE 0)   * 1 FETCH (BODY ("TEXT" "PLAIN" NIL NIL NIL "7BIT" 0 0)   * 1 FETCH (BODYSTRUCTURE ("TEXT" "PLAIN" NIL NIL NIL "7BIT" 0 0)   * 1 FETCH (BODY[<section>] "")   * 1 FETCH (BODY[<section>]<partial> "")   In some cases, a client may not be able to distinguish between "NIL   FETCH responses" received because a message was expunged and those   received because the data actually was NIL.  For example, a  * 5   FETCH (FLAGS ()) response could be received if no flags were set on   message 5, or because message 5 was expunged. In a case of potential   ambiguity, the client SHOULD issue a command such as NOOP to force   the sending of the EXPUNGE responses to resolve any ambiguity.   Example:  (Building upon the scenario outlined in 4.1.)   <Client #2 attempts to access a mix of expunged and non-expunged   messages.  Normal data is returned for non-expunged message, "NIL   FETCH responses" are returned for expunged messages>Gahrns                       Informational                      [Page 8]

RFC 2180         IMAP4 Multi-Accessed Mailbox Practice         July 1997             C2: B002 FETCH 3:5 ENVELOPE             S2: * 3 FETCH ENVELOPE . . . (ENVELOPE info returned)             S2: * 4 FETCH ENVELOPE (NIL NIL NIL NIL NIL NIL NIL NIL                   NIL NIL)             S2: * 5 FETCH ENVELOPE (NIL NIL NIL NIL NIL NIL NIL NIL                   NIL NIL)             S2: B002 OK FETCH Completed   <Client #2 attempts to FETCH only expunged messages and receives a   tagged NO response>             C2: B002 FETCH 4:7 ENVELOPE             S2: B002 NO Messages 4:7 have been expunged.4.1.4 To avoid the situation altogether, the server MAY fail the      EXPUNGE of a multi-accessed mailbox   In some cases, this behavior may not be practical.  For example, if a   large number of clients are accessing a shared mailbox, the window in   which no clients have the mailbox accessed may be small or non-   existent, effectively rendering the message unexpungeable.4.2. Storing of expunged messages   Following are some strategies an IMAP server may choose to use when   dealing with a STORE command on expunged messages.4.2.1 If the ".SILENT" suffix is used, and the STORE completed      successfully for all the non-expunged messages, the server SHOULD      return a tagged OK.   Example:  (Building upon the scenario outlined in 4.1.)   <Client #2 tries to silently STORE flags on expunged and non-   expunged messages.  The server sets the flags on the non-expunged   messages and returns OK>             C2: B001 STORE 1:7 +FLAGS.SILENT (\SEEN)             S2: B001 OKGahrns                       Informational                      [Page 9]

RFC 2180         IMAP4 Multi-Accessed Mailbox Practice         July 19974.2.2. If the ".SILENT" suffix is not used, and only expunged messages       are referenced, the server SHOULD return only a tagged NO.   Example:  (Building upon the scenario outlined in 4.1.)   <Client #2 tries to STORE flags only on expunged messages>             C2: B001 STORE 5:7 +FLAGS (\SEEN)             S2: B001 NO  Messages have been expunged4.2.3. If the ".SILENT" suffix is not used, and a mixture of expunged       and non-expunged messages are referenced, the server MAY set the       flags and return a FETCH response for the non-expunged messages       along with a tagged NO.   After receiving a tagged NO STORE response, the client SHOULD issue a   NOOP command so that it will be informed of any pending EXPUNGE   responses.  The client may then either reissue the failed STORE   command, or by examining the EXPUNGE responses from the NOOP and   FETCH responses from the STORE, determine that the STORE failed   because of pending expunges.   Example:  (Building upon the scenario outlined in 4.1.)   <Client #2 tries to STORE flags on a mixture of expunged and non-   expunged messages>             C2: B001 STORE 1:7 +FLAGS (\SEEN)             S2: * FETCH 1 FLAGS (\SEEN)             S2: * FETCH 2 FLAGS (\SEEN)             S2: * FETCH 3 FLAGS (\SEEN)             S2: B001 NO Some of the messages no longer exist.             C2: B002 NOOP             S2: * 4 EXPUNGE             S2: * 4 EXPUNGE             S2: * 4 EXPUNGE             S2: * 4 EXPUNGE             S2: * 3 EXISTS             S2: B002 OK NOOP Completed.   <By receiving FETCH responses for messages 1:3, and an EXPUNGE   response that indicates messages 4:7 have been expunged, the client   does not need to re-issue the STORE>Gahrns                       Informational                     [Page 10]

RFC 2180         IMAP4 Multi-Accessed Mailbox Practice         July 19974.2.4. If the ".SILENT" suffix is not used, and a mixture of expunged       and non-expunged messages are referenced, the server MAY return       an untagged NO and not set any flags.   After receiving a tagged NO STORE response, the client SHOULD issue a   NOOP command so that it will be informed of any pending EXPUNGE   responses.  The client would then re-issue the STORE command after   updating its message list per any EXPUNGE response.   If a large number of clients are accessing a shared mailbox, the   window in which there are no pending expunges may be small or non-   existent, effectively disallowing a client from setting the flags on   all messages at once.   Example:  (Building upon the scenario outlined in 4.1.)   <Client #2 tries to STORE flags on a mixture of expunged and non-   expunged messages>             C2: B001 STORE 1:7 +FLAGS (\SEEN)             S2: B001 NO  Some of the messages no longer exist.   <Client #2 issues a NOOP to be informed of the EXPUNGED messages>             C2: B002 NOOP             S2: * 4 EXPUNGE             S2: * 4 EXPUNGE             S2: * 4 EXPUNGE             S2: * 4 EXPUNGE             S2: * 3 EXISTS             S2: B002 OK NOOP Completed.   <Client #2 updates its message list and re-issues the STORE on only   those messages that have not been expunged>             C2: B003 STORE 1:3 +FLAGS (\SEEN) S2: * FETCH 1 FLAGS             (\SEEN) S2: * FETCH 2 FLAGS (\SEEN) S2: * FETCH 3 FLAGS             (\SEEN) S2: B003 OK  STORE Completed4.3. Searching of expunged messages   A server MAY simply not return a search response for messages that   have been expunged and it has not been able to inform the client   about.  If a client was expecting a particular message to be returned   in a search result, and it was not, the client SHOULD issue a NOOP   command to see if the message was expunged by another client.Gahrns                       Informational                     [Page 11]

RFC 2180         IMAP4 Multi-Accessed Mailbox Practice         July 19974.4 Copying of expunged messages   COPY is the only IMAP4 sequence number command that is safe to allow   an EXPUNGE response on.  This is because a client is not permitted to   cascade several COPY commands together. A client is required to wait   and confirm that the copy worked before issuing another one.4.4.1 The server MAY disallow the COPY of messages in a multi-access      mailbox that contains expunged messages.   Pending EXPUNGE response(s) MUST be returned to the COPY command.   Example:             C: A001 COPY 2,4,6,8 FRED             S: * 4 EXPUNGE             S: A001 NO COPY rejected, because some of the requested                messages were expunged   Note: Non of the above messages are copied because if a COPY command   is unsuccessful, the server MUST restore the destination mailbox to   its state before the COPY attempt.4.4.2 The server MAY allow the COPY of messages in a multi-access      mailbox that contains expunged messages.   Pending EXPUNGE response(s) MUST be returned to the COPY command.   Messages that are copied are messages corresponding to sequence   numbers before any EXPUNGE response.   Example:             C: A001 COPY 2,4,6,8 FRED             S: * 3 EXPUNGE             S: A001 OK COPY completed   In the above example, the messages that are copied to FRED are   messages 2,4,6,8 at the start of the COPY command.  These are   equivalent to messages 2,3,5,7 at the end of the COPY command.  The   EXPUNGE response can't take place until after the messages from the   COPY command are identified (because of the "no expunge while no   commands in progress" rule).Gahrns                       Informational                     [Page 12]

RFC 2180         IMAP4 Multi-Accessed Mailbox Practice         July 1997   Example:             C: A001 COPY 2,4,6,8 FRED             S: * 4 EXPUNGE             S: A001 OK COPY completed   In the above example, message 4 was copied before it was expunged,   and MUST appear in the destination mailbox FRED.5. Security Considerations   This document describes behavior of servers that use the IMAP4   protocol, and as such, has the same security considerations as   described in [RFC-2060].   In particular, some described server behavior does not allow for the   immediate deletion of information when a mailbox is accessed by   multiple clients.  This may be a consideration when dealing with   sensitive information where immediate deletion would be preferred.6. References   [RFC-2060], Crispin, M., "Internet Message Access Protocol - Version   4rev1",RFC 2060, University of Washington, December 1996.   [RFC-2119], Bradner, S., "Key words for use in RFCs to Indicate   Requirement Levels",RFC 2119, Harvard University, March 1997.7.  Acknowledgments   This document is the result of discussions on the IMAP4 mailing list   and is meant to reflect consensus of this group.  In particular,   Raymond Cheng, Mark Crispin, Jim Evans, Erik Forsberg, Steve Hole,   Mark Keasling, Barry Leiba, Syd Logan, John Mani, Pat Moran, Larry   Osterman, Chris Newman, Bart Schaefer, Vladimir Vulovic, and Jack De   Winter were active participants in this discussion or made   suggestions to this document.Gahrns                       Informational                     [Page 13]

RFC 2180         IMAP4 Multi-Accessed Mailbox Practice         July 19978. Author's Address   Mike Gahrns   Microsoft   One Microsoft Way   Redmond, WA, 98072   Phone: (206) 936-9833   EMail: mikega@microsoft.comGahrns                       Informational                     [Page 14]